1. Field of the Invention
The present invention relates to a process for producing fine non-oxide powder from an alkoxide such as a silicon or aluminum alkoxide. More particularly, the present invention relates to a process for producing fine powder of silicon or aluminum nitride or a fine powder mixture of silicon nitride and silicon carbide, which is useful as a starting material for a sintered product.
2. Description of the Prior Art
A sintered product of silicon nitride has high strength and high thermal shock resistance, and it is expected to be applied to heat resistant machine parts such as automobile engine parts or gas turbine parts. If this is put into practice, the engines or turbines can be operated without requiring a cooling system, which will greatly contribute to the saving of energy.
A sintered product of aluminum nitride has high corrosion resistance and good heat conductivity, and it is expected to be applied to machine parts for metal refining or heat-resistant substrates for semiconductors.
A sintered product of silicon carbide has high strength and good heat conductivity. However, the sintering of silicon carbide requires a high temperature of at least 2000.degree. C. On the other hand, silicon nitride can be sintered at a temperature of 1750.degree. C. or lower, and the sintered product has high strength. However, the heat conductivity of the sintered product of silicon nitride is low as compared with the sintered product of silicon carbide. Whereas, a sintered product of a mixture of silicon nitride and silicon carbide has high strength and good heat conductivity. Such a sintered product is useful for high temperature heat exchangers or heat resistant substrates for semiconductors.
For the production of fine powder of silicon nitride, there have been known:
(1) a method in which metal silicon is heated in a nitrogen stream at a temperature of from 1300.degree. to 1400.degree. C.;
(2) a method in which a powder mixture of silica and carbon is heated in a nitrogen stream at a temperature of from 1400.degree. to 1500.degree. C.
(3) a method in which a Si-containing gas and a carbon-containing gas are reacted in a gaseous phase at a temperature of from 1300.degree. to 1600.degree. C.; and
(4) a method in which SiCl.sub.4 and NH.sub.3 are reacted in a solvent at a low temperature, and the resulting Si(NH).sub.2 is heated.
The above method (1) is used on an industrial scale. However, it has drawbacks that it is necessary to use iron or manganese as a catalyst for nitriding, and it is difficult to obtain highly pure powder.
According to the method (2), it is possible to obtain highly pure silicon nitride powder by using highly pure silica. However, it is difficult to obtain a uniform mixture of silica and carbon, and accordingly, a great amount of carbon as much as from 3 to 10 times the theoretical amount is required to complete the reaction. Therefore, it is necessary to remove excess carbon after the reaction by heating the reaction product in air at a temperature of from 700.degree. to 800.degree. C. This brings about a drawback that the surface of silicon nitride powder is oxidized.
Thus, according to the methods (1) and (2), the silicon nitride powder contains metal or non-metal (oxygen) as impurities. If such powder is sintered, the impurities tend to precipitate at the boundaries of silicon nitride particles and thus impair the high temperature strength, oxidation resistance and corrosion resistance.
According to the methods (3) and (4), it is possible to obtain highly pure fine powder. However, such fine powder particles are likely to firmly bond one another to form large particles (agglomerates). Such agglomerates act as large particles during the sintering, and accordingly they have the same drawbacks as those of large particles. Further, the powder is expensive.
For the production of aluminum nitride, there have been known:
(1) a direct-nitriding method in which aluminum is heated in nitrogen;
(2) a reduction-nitriding method in which a mixture of alumina and carbon is heated in nitrogen at a temperature of from 1600.degree. to 2000.degree. C.; and
(3) a gaseous phase reaction method in which an aluminum compound (gas) and nitrogen or ammonia are reacted.
The above method (1) has an advantage that the aluminum nitride powder is produced inexpensively by the direct nitriding method. However, it is difficult to completely nitride highly pure aluminum metal, and it is necessary to add other metals as a nitriding catalyst. Consequently, the metal of the added nitriding catalyst will remain in the formed aluminum nitride powder, and it is difficult to obtain highly pure aluminum nitride powder.
The reduction-nitriding method (2) does not require a catalyst for the reaction, and accordingly, it is possible to obtain highly pure powder. However, it is difficult to uniformly mix the starting material powder, and it is necessary to add a large excess amount of carbon powder in order to complete the reaction. Accordingly, it will be required to remove the remaining carbon after the reaction, by after-treatment such as heating in air. For the removal of a large amount of carbon, heating for a long period of time is required, whereby aluminum nitride is likely to be oxidized.
According to the gaseous phase reaction method (3), it is possible to obtain highly pure fine powder. However, the starting material is expensive and a large scale apparatus will be required. Thus, the method has a drawback that the resulting powder is expensive.
For the production of a powder mixture of silicon nitride and silicon carbide, there have been known:
(1) a method in which powders of silicon nitride and silicon carbide having a particle size of at most 1 .mu.m are mixed; and
(2) a method in which a mixture of silica and carbon is heated in an atmosphere of nitrogen and carbon monoxide.
However, according to the method (1), it is difficult to obtain a uniform mixture. Accordingly, the sinterability of the powder will be inadequate, and there is a drawback that it is difficult to obtain a high density sintered product even by hot-pressing.
According to the method (2), it is necessary to add a great amount of carbon in order to complete the reaction. Consequently, a substantial amount of carbon will remain in the powder mixture. Such a mixture hardly sinters even when a sintering aid is added. Therefore, it is necessary to conduct after-treatment for the removal of the excess carbon e.g. by heating in air. This not only makes the operation cumbersome, but also adds to the cost.